Tires: Cupping?, chevy venture, rear shocks

Question
Thanks for the answer.  
  I think we're sort of saying the same thing but in different ways, i.e. glass half empty, glass half full.  If I DON'T rotate, all the particular kind of wear at a tire's location gets exaggerated or another way to look at it is that if I DO rotate, the wear due to misalignment in different locations gets distributed evenly over all the tires.  The way I see it, they're the same idea said in different ways.  
  However, my concern isn't with that, but with the solution to the problem which is exaggerated by lack of rotation, namely misalignment.  I've not been able to get a satisfactory answer from the establisments I've called.  Can the rear wheels on this van ('01 Chevy Venture) be aligned?  The fronts are worn, but are pretty much worn uniformly, so I don't think there is a big problem there.  If I can't align the rear, then I guess I'll have to make sure that the rotation is done so as to distribute the pain.

Thanks again

Joe
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Followup To
Question -
Hi,
  I have a 2001 Chevy Venture Minivan with 38K miles.  The REAR tires have a problem in that the inside half of the tire looks like you've taken scoops of rubber off.  The tires are the original equipment (General XP 2000).  They make lots of noise when you drive (sounds like a "rrr-rrr" sound that varies with speed and gets louder on braking) and I'm getting ready to replace them in the next few days.  My concern is whether this is a problem with the tires or the van.  I was told by a friend that it may be the rear shocks.  I had those checked (auto leveling from the factory) at the dealer and they tell me they're fine.  When I asked the dealer to explain the wear on the tires, he said it was due to my not rotating the tires.  I don't buy that since its my understanding that rotating the tires simply makes the wear uniform across all 4.
Thanks for any help you can provide.
Answer -
Joe,

This type of wear is common on your vehicle.  It's caused by misalignment and aggravated by insufficient rotation and insufficient inflation pressure.

But you've got the bit about rotation backwards.  Each wheel position has its own characteristic wear pattern that's determined by the vehicle (suspension geometry, steering geometry, braking action, etc.), the type of driving you do (road surface texture, hills, curves, amount and sharpness of the turns, etc.), and your driving style.  If the tire is left in that position, the wear gets exaggerated (irregular).

Rotation causes the tire to develop a different - and usually counterbalanced - wear pattern.

Put another way, rotation doesn't generate even wear, it counteracts irregular and uneven wear.

Hope this helps.  

Answer
Joe,

Let me answer the BIG question first:

There is something in the suspension of the vehicle that is going to cause the rear tires to develop irregular wear.  I've seen it many times, but I've never been in the position to try to correct it, so I can't really advise as to what to do, if anything.  My gut feeling is that there is not a "fix".

Now to the issue of rotation:  I think we are NOT saying the same thing in different ways.  It sounds like you believe that if a vehicle is in alignment that the result would be even wear in every wheel position.  This is impossible.

Vehicle suspension design is a compromise and because it is a compromise, some characteristic isn't the best it could be.  Sometimes it's tire wear; sometimes it's space utilization, sometimes it's directional stability.  Let me give an example:

Directional stability is very important to the feel of the vehicle.  Put another way - if the vehicle doesn't "feel" like it's going the direction the driver wants it to go, then the driver isn't going to like it.  One of the ways to accomplish straight line stability is to give the tires a bit of toe in.  By definition, this is misalignment.  Since the bushings are made of some non-rigid material (rubber, urethane, etc.),  the toe in spec is usually a bit more than the minimum needed.  This prevents the toe from ever being toe out - an unstable situation.

With even the barest minimum of toe in, some irregular wear is going to develop - the question is when will this become a problem.  By rotating the tire to a different postion, this wear pattern being developed is replaced by a different wear pattern.  Most of the time the new wear pattern is complimentary - that is the new wear pattern cancels out the old one.  

Another aspect of this is that it is impossible to get a "perfect" alignment.  By that I mean that the suspension geometry and the steering geometry can not be designed in such a way that it is "perfect" for every situation.  This means that even if the vehicle meets the alignment "specs", there are driving situations where this setting isn't the most optimal.

In your case, the front wheel drive causes such rapid wear on the front tires that only a severe misalignment is going to develop irregular wear.  The net effect here is that the front tires will wear out long before they develop irregular wear.  This of course is a complimentary position to the rear tires, which are trying to develop irregular wear, albeit relatively slowly, but much more rapidly in comparison the overall rate of wear.  

One other thing which I think will help clarify the picture.  Irregular wear develops on an exponential curve.  That is, it starts off slowly, but as it develops, it develops more rapidly.  Put another way, the difference between slight and moderate irregular wear might take 10,000 miles, but the difference between moderate to severe might only take 3,000 miles.  

So rotating the tires on a regular basis is a good way to prevent these things from becoming perceivable.

Hope this helps.